The recent miniaturization and increased levels of integration of semiconductor integrated circuits has led to a demand for increases in the resolutions of semiconductor exposure devices. A semiconductor exposure device is referred to as an “exposure device” hereinafter. Accordingly, reduction of the wavelengths of light emitted from exposure light sources is improving. Typically, gas laser apparatuses are used as exposure light sources instead of conventional mercury lamps. For example, a KrF excimer laser apparatus that outputs an ultraviolet laser beam at a wavelength of 248 nm and an ArF excimer laser apparatus that outputs an ultraviolet laser beam at a wavelength of 193 nm are used as gas laser apparatuses for exposure.
As a current exposure technology, immersion exposure has been put into practical use. In the immersion exposure, a gap between an exposure lens in an exposure apparatus and a wafer is filled with fluid, and an apparent wavelength of the exposure light source is shortened by changing the refractive index of the gap. In a case where immersion exposure is performed using an ArF excimer laser apparatus as an exposure light source, a wafer is irradiated with ultraviolet light whose wavelength in water is equivalent to 134 nm. This technique is referred to as “ArF immersion exposure”. ArF immersion exposure is also referred to as “ArF immersion lithography”.
Spectrum line widths of KrF and ArF excimer laser apparatuses in natural oscillation amplitudes are as wide as approximately 350 to 400 pm. This causes a chromatic aberration of a laser beam (ultraviolet light) that is subjected to reduced projection onto a wafer by a projection lens on the side of an exposure device, thus causing a reduction in resolving power. Therefore, a spectrum line width of a laser beam that is outputted from a gas laser apparatus needs to be narrowed to such an extent that the chromatic aberration can be ignored. The spectrum line width is also referred to as “spectrum width”. To narrow the spectrum width, a laser resonator of a gas laser apparatus is provided with a line narrow module having a line narrow element. The line narrow element may be an etalon, a grating, or the like. A laser apparatus whose spectrum width is narrowed in this way is referred to as “narrow band laser apparatus”.